If you use Apache Commons Lang, the easiest way is

RandomStringUtils.random(20, 0, 0, true, true, null, new SecureRandom());

bytes.toString(); is wrong, try using Arrays.toString(bytes) - or new String(bytes) if you want to convert it to a String.

You can turn the bytes in a BigInteger and do a base 62 (=10+26+26) conversion on it.

// Digits listed, so maybe look-alike chars (zero and oh) can be handled.
private static final String digits = "0123...ABC...abc...z";

String humanReadable(byte[] bytes) {

    // Ensure that we have a byte array which is a positive big-endian.
    if (bytes.length != 0 && bytes[0] < 0) {
        byte[] ubytes = new byte[bytes.length + 1];
        System.arraycopy(bytes, 0, ubytes, 1, bytes.length);
        bytes = ubytes;
    }

    BigInteger n = new BigInteger(bytes);
    StringBuilder sb = new StringBuilder(48);
    final BigInteger DIGIT_COUNT = BigInteger.valueOf(digits.length());
    while (n.signum() != 0) {
        int index = n.mod(DIGITS).intValue();
        sb.append(digits.charAt(index));
        n = n.div(DIGITS);
    }
    return sb.toString();
}